This invention relates to the field of two-way communications and more particularly to the capability of remote channel selection.
The development of mobile communications systems began with vehicles having only a receiver for receiving information from a base station, then quickly progressed to a two-way system. Later, portable units were added to the mobile units in the vehicles to enable users to remain in contact with the base station, whether in or out of the vehicle. The next major step was the addition of a repeater unit to the vehicle, whereby transmission from the relatively low powered portable unit is picked up by the mobile unit and retransmitted to the base station at a higher power on another frequency. Transmission back from the base station could be on this second frequency or still another frequency.
In public safety work, it frequently happens that several vehicles in a system are involved at one location, thus prioritization schemes were developed to ensure that one and only one mobile/repeater unit would repeat out all portable-to-base transmission at that location. Two such repeater prioritizing systems are disclosed in U.S. Pat. Nos. 4,056,780 and 4,056,779, both assigned to the same assignee as is the present invention. Repeater prioritization may be used with the present channel priority encoder, but the two systems are completely distinct.
Another, and parallel, development was the addition of multiple channel capability to mobile units. Thus, for example, in a state police system, a vehicle could travel throughout the state and be able to contact any base station within the system. Multiple channels are also utilized in other ways within a single area and it became desirable for a portable user to be able to change channels without having return to his vehicle to reset a channel selector. Under various circumstances, it may be desirable to have only the mobile-to-base channel selectable (transmitter steering), or to select the same channel for two-way communication with the base station. In the latter case, it may be desirable to latch-in the selection (transmitter/receiver latching), or to hold the channel selection until the end of a conversation and, then return the repeater to a "home" channel (transmitter/receiver steering).